From 942bea65d9b4a4e97dd765c93748d3ffee2076dd Mon Sep 17 00:00:00 2001
From: Greg Price <greg@zulip.com>
Date: Mon, 27 Nov 2023 19:29:59 -0800
Subject: [PATCH] msglist: Optimize the _messageVisible check to avoid copying
 where possible

---
 lib/model/message_list.dart | 23 ++++++++++++++++++++++-
 1 file changed, 22 insertions(+), 1 deletion(-)

diff --git a/lib/model/message_list.dart b/lib/model/message_list.dart
index 2ef95771e4f..f9f65e0aab3 100644
--- a/lib/model/message_list.dart
+++ b/lib/model/message_list.dart
@@ -312,6 +312,8 @@ class MessageListView with ChangeNotifier, _MessageSequence {
   ///
   /// This depends in particular on whether the message is muted in
   /// one way or another.
+  ///
+  /// See also [_allMessagesVisible].
   bool _messageVisible(Message message) {
     switch (narrow) {
       case AllMessagesNarrow():
@@ -332,6 +334,21 @@ class MessageListView with ChangeNotifier, _MessageSequence {
     }
   }
 
+  /// Whether [_messageVisible] is true for all possible messages.
+  ///
+  /// This is useful for an optimization.
+  bool get _allMessagesVisible {
+    switch (narrow) {
+      case AllMessagesNarrow():
+      case StreamNarrow():
+        return false;
+
+      case TopicNarrow():
+      case DmNarrow():
+        return true;
+    }
+  }
+
   /// Fetch messages, starting from scratch.
   Future<void> fetchInitial() async {
     // TODO(#80): fetch from anchor firstUnread, instead of newest
@@ -380,7 +397,11 @@ class MessageListView with ChangeNotifier, _MessageSequence {
         result.messages.removeLast();
       }
 
-      _insertAllMessages(0, result.messages.where(_messageVisible));
+      final fetchedMessages = _allMessagesVisible
+        ? result.messages // Avoid unnecessarily copying the list.
+        : result.messages.where(_messageVisible);
+
+      _insertAllMessages(0, fetchedMessages);
       _haveOldest = result.foundOldest;
     } finally {
       _fetchingOlder = false;